1. Field of the Invention
The present invention relates to a laminated varistor, and more particularly, to a laminated varistor including a semiconducting ceramic which comprises ZnO as a primary component and at least Bi oxide as a secondary component, and internal electrodes which predominantly comprise Pt. The present invention also relates to a method for producing the laminated varistor. The invention further relates to a material and a paste for producing internal electrodes of a varistor, the material and the paste having reduced Pd content.
2. Background Art
Recently, the field of electronic elements employed in communication devices and the like has seen rapid progress in miniaturization and in lowering of driving voltage. Without exception, varistors must be miniaturized so as to enhance the packaging density thereof and must be driven at low voltage. In order to satisfy these demands, Japanese Patent Application Laid-Open (kokai) No. 5-283208 discloses a laminated varistor.
The above-mentioned laminated varistor includes a laminate formed by laminating alternating layers of semiconducting ceramic which contains ZnO as a primary component and at least Bi oxide as a secondary component, and internal electrodes which predominantly comprise Pt; and then firing the laminated product at 1000.degree. C. or higher, wherein external electrodes are formed on two side faces of the laminate such that each of the internal electrodes has one end exposed to one of the side faces so as to establish electric contact with one of the external electrodes. In the laminated varistor, semiconducting ceramic grains are grown such that the grain size is approximately equal to the distance between adjacent internal electrodes. Thus, voltage nonlinearity of the varistor is basically developed from interface between one semiconducting ceramic layer and one internal electrode.
However, laminated varistors having conventional internal electrodes formed of Pt have disadvantageously variable electric characteristics, i.e., irregular variation of voltage nonlinearity and withstand voltage against electrostatic discharge.
These variable electric characteristics are attributed to formation of Pd-Bi-O having high electric resistance. Pd is an inevitable impurity in the Pt and originates from materials for producing internal electrodes of varistors.
Although a varistor including internal electrodes formed of Pt containing no Pd theoretically exhibits excellent electric characteristics, the sintering temperature of such internal electrodes is as high as 1200.degree. C., i.e., an advantageous low-temperature firing is not attained.